Supplemental inflatable restraints (SIR) or air bags are used in automotive vehicles to deploy upon detecting an impact to cushion the driver or other occupant from injury as a result of the impact. Such systems include a sensing and deployment module, a squib controlled by the module to fire when an impact is sensed, an inflatable bag, and an inflating device triggered by the squib to deploy the bag. The squib is outside the module in an external deployment path. The module includes a diagnostic capability for checking or monitoring the resistance of the external path, thereby verifying that the squib is operable.
A conventional diagnostic circuit for measuring the resistance in the external path of a SIR is shown in FIG. 1. The external path 12 containing the squib 14 is coupled by connectors 16 to the sensing and deployment module 18 which includes the diagnostic circuit 20. The diagnostic circuit is supplied by the vehicle battery 22 and the voltage is increased and regulated by a boost supply 24.
A constant current source 26 is connected from the boost supply 24 to one end of the external path 12 via a connector 16, while the other connector 16 connects the path 12 through a current limiting device comprising resistor 28 and a transistor switch 30 to ground. The inputs of a differential amplifier 32 are connected to either end of the external path 12 adjacent the connectors 16, and the amplifier output is fed through an A/D converter 34 and the resulting digital data is fed to a microprocessor 36. The switch 30 is controlled directly or indirectly by the microprocessor 36 via line 38.
The circuit operates when the transistor switch 30 is turned on, allowing the current output of the constant current source 26 to pass through the external path 12 to ground. The voltage developed across the external path, which is indicative of the integrity or fitness of the path, is amplified and sampled by the microprocessor for analysis. The current of the current source 26 is chosen to be small enough, say 150 mA, to impose no danger of either firing the squib or heating it enough to cause chemical deterioration.
When the switch 30 is turned on for a test, the fixed current flows through the external path 12 and the voltage generated across the path is amplified, digitized and then assessed by the microprocessor which is programmed to determine the condition of the external path. The resistor 28 is of the order of 100 ohms and is necessary to limit current to a harmless value in the event that battery voltage is inadvertently applied to the high end of the external path 12 while the transistor switch 30 is conducting.
The test current of 150 mA will cause a voltage drop of 15 volts in the resistor 28 and additional voltage drops in current source 26, the squib 14 and the transistor 30 for a total voltage drop of at least 18 volts. The vehicle battery voltage, having a range of 9 to 16 volts is insufficient to directly supply 18 volts so that the boost supply 24, affording 18 to 36 volts, is necessary.
It is desirable to avoid the expense of the boost supply and thus an alternate resistance measurement circuit is here proposed.